Intel 8088, 16-bit breadboard computer

Series of tutorial-style videos where I build a 16-bit x86 Intel 8088 computer on breadboards explaining on the way the internals of this famous processor. You will see how to interface with the 8088 when I add the memory chips and other devices to the breadboards. All the signals of the CPU buses are explained when I step through the programme including all the details of the interrupt handling in this processor. Later in this series I explain how to programme the x86 using both the assembler and higher-level languages like C or C++ and how to mix them in one programme. The final goal is to write a simple video game for our computer.

Future plans

Currently we are well into the series. There is a few more videos planned in order to fulfil my promise to write and play a simple video game on my computer.
After I’ve achieved it, I hope to continue this series by adding some non-essential extensions or changing the existing hardware to show that this computer may be built in many different ways. I already have a few ideas to try – couple of them taken from my viewers’ comments – thank you for all of them!

Resources

All the source code and schematics for this series can be found on GitHub:
https://github.com/sladorsoft/Intel8088OnBreadboard

Software used in this series:

• NASM assembler
• GCC IA-16 toolchain
• Make for Windows (optional if use Windows)
• Visual Studio Code (optional)
• KiCad 6 (for schematics)

Part 1 – First Run

In this first video of the series I:

• set my goals for the series and talk about what I am going to do
• describe the 8088/8086 and show the differences between the two
• put the 8088 on a breadboard and start connecting all relevant pins explaining all the input and output signals of the CPU
• build a simple contraption to drive the CLOCK and RESET signals
• talk about 8086 address space and how it addresses the memory
• explain how the processor starts executing machine code after reset
• connect LEDs to all data/address and control pins to visualise their states
• start the CPU and analyse its behaviour step by step after reset

Part 2 – First Real Programme

In this video you can see how I:

• add latches to store the address signals from the multiplexed data/address pins
• connect resistors to drive the data bus and simulate the NOP (no-operation) machine code for the CPU to fetch and execute while clock signal step by step
• add the proper flash memory and with a simple programme
• talk about 8086 registers and how it divides the 1MB memory space into 64kB segments
• explain in details the test programme in the flash memory
• run the programme on our breadboard computer and visualise the CPUs address and data buses on the LEDs
• allow the programme to show its calculations on the data bus LEDs

Part 3 – Memory map and interrupts

In this video I:

• talk about the memory map of our computer
• build an address decoder using logic gates, to activate two different memory chips depending on the address the CPU wishes to access
• add a RAM memory chip and connect both memories to the address decoder
• talk about interrupts in 8086 and explain the Interrupt Vector Table at the bottom of the RAM
• introduce the Programmable Interrupt Controller (8259) and its role in interrupt handling
• explain the concept of the processor’s stack and its importance in the context of interrupt handling
• go into details of the programme used to initialise the processor and the handler of the Non-Maskable Interrupt
• raise the MNI interrupt (using a wire) and analyse the CPUs behaviour in details

Part 4 – More interrupts, clock generator and LCD

In this video you can join me to see how I:

• add a new chip (74244) with 8 buffers to simulate the PIC’s response to the interrupt request (without actually using the 8259)
• write a new programme with a new interrupt handler for the INTR (maskable) requests
• raise an interrupt using a wire on the INTR pin and, like previously, analyse the CPUs behaviour step by step
• add an I/O address decoder using 74138 chip which allows us to connect 8 different I/O devices
• add a Text LCD as our first device in the I/O address space, including logic required to access it from the CPU
• programme a set of functions to talk to the LCD and test it by running the computer step by step
• replace the manual clock with the proper crystal square wave generator using the Intel 8284 chip
• test the whole computer and the LCD again – this time by using high frequency clock generator

Part 5 – Calling conventions and single-step execution

This video is exclusively devoted to programming our breadboard computer and the theory behind it. You can see how I:

• tidy up the last programme for the Text LCD testing
• explain calling conventions allowing to mix code from different languages
• talk about differences between functions compiled in different languages
• how to write functions in assembler, which can be called from C and which can call other C functions properly
• write functions both in assembler and C to support the Text LCD
• create a function to display values of all Intel 8086 registers on the LCD
• explain the breakpoint interrupt (INT 3)
• write the breakpoint handler to collect the full state of all CPU’s registers and call the function to display them written in C
• discuss in details the FLAGS register of the 8086
• show how to switch the 8086 into the single-step mode
• write the single-step interrupt handler (INT 1) and display the registers’ states on the LCD

Part 6 – More I/O devices: Interrupt controller and timers

In this video I add more peripherals and programme them. Here is what happens step by step:

• reactivate the 74244 octal buffers to act as a read-only register reflecting the states of the new push buttons
• add functions to read the button states to allow single-step full functionality
• add the PIC 8259 – Programmable Interrupt Controller
• initialise the PIC and test by raising an interrupt manually while single-stepping
• add the programmable timers – the 8254
• set-up the timer to raise a regular system tick interrupt 100 times a second
• test the whole thing
• talk a little bit more about flags in the FLAGS register

Part 7 – CPU upgrade

In this video I build a new computer using 80188 processor. This is what you will see in this video:

• 80188 introduction. Differences between multiple versions
• peripherals in the 80188
• similarities and differences of the pins in both 8088 and 80188
• changes required in software to handle the built-in peripherals
• testing the new computer
• both computers running the same programme side by side

Part 8 – Intel 80188 computer on a PCB

In this video I design a PCB for the 80188 computer from the last video and add programmable I/O ports – the 8255.
This is what you will see:

• the design of the PCB for my 80188 computer
• discussion about using the timers in the 80188
• testing new computer soldered on the PCB with the text LCD
• introduction of the programmable I/O ports: the 8255
• testing the 8255 using the LCD connected via its I/O pins